The heteromeric transporter OST Alpha OST Beta facilitates the transport of bile and other steroid solutes across the basolateral epithelial cell membrane of intestine, liver, testis, kidney and adrenal gland. OST Alpha OST Beta expression is induced by bile acids through ligand-dependent transactivation of their genes by FXR (Farnesoid X-activated receptor). This genetic regulation suggests that in response to changes in intracellular bile acid levels, bile acids adjust the rate of their own efflux from enterocytes. OST Beta is a 128 amino acid single-pass transmembrane protein that requires OST Alpha to localize to the plasma membrane. Coexpression of OST Alpha and OST Beta is also required to convert the OST Alpha subunit to a mature glycosylated endoglycosidase H-resistant form, suggesting that co-expression facilitates trafficking of OST Alpha through the golgi apparatus. Though widely expressed, OST Beta is present at highest levels in ileum.